We’re blogging through St. Thomas Aquinas’ Compendium Theologiae, sometimes called his Shorter Summa. Find the previous posts here.
In the previous chapter, Thomas showed that God’s essence can not be represented as a genus and a specific difference, and thus God is not a species within a genus. Now he goes on to show that God isn’t a genus, either. It does not strike me as likely that He could be, mind you, but Thomas is always thorough.
Let me note again: these terms, genus and species, are not being used here as a metaphor, or as an analogy to the familiar biological usage of the same terms. Rather, these are the classic philosophical and metaphysical terms for understanding the nature of being, and appear at least as far back as the 3rd Century in Porphyry the neo-Platonist’s Tree of Being; and the concepts are present in Aristotle’s Categories.
We go on to show that God cannot be a genus. What a thing is, but not that it is, comes from its genus; the thing is established in its proper existence by specific differences. But that which God is, is very existence itself. Therefore He cannot be a genus.
What does he mean when he says, a “thing is established in its proper existence by specific differences”?
Look at it like this: suppose you’re going to build a chair. Before you can start (or, at least, before you can finish) you have to know everything about the design and construction of the chair. What materials is it made of? What are its dimensions? Is it a straight chair or an overstuffed chair?
Those last two choices are specific differences: not just a chair, but a straight chair. Or an overstuffed chair. Both are chairs, but simply saying “chair” isn’t enough; we need to know more.
According to Porphyry’s tree, a dog is: a body (it takes up space) that takes in nourishment and grows (it’s a living thing) that moves of itself (it’s an animal, not a plant) that is a dog (I.e., not a cat, a horse, a pig, or a person).Now here’s the point: if you tell me something is an “animal”, that tells me quite a lot about it; but it doesn’t tell me everything. Animal is a pure genus; there are many things in the world of which I can say, “That’s an animal”, but nothing in the world of which I can say, “That’s just an animal.”
In short, Thomas is saying that genera exist in concept, as an abstraction from real things, but that you need to add one or more specific differences before you can get to the essence of a concrete thing. A bare genus cannot exist by itself. But since God is existence itself, then, He must not be a genus.
Moreover, every genus is divided by some differences. But no differences can be apprehended in very existence itself. For differences do not share in genus except indirectly, so far as the species that are constituted by differences share in a genus. But there cannot be any difference that does not share in existence, since non-being is not the specific difference of anything. Accordingly God cannot be a genus predicated of a number of species.
What Thomas is getting at, here, is that existence is unlike other things we might say about a thing. Aristotle in his Categories and Porphyry in his Tree of Being enumerated the various ways a thing can be said to be; but all of them are things we can talk about in concept without actually having concrete beings in view. I can talk about dogs all day long without seeing or touching a real dog. To get a real dog, this dog here in front of me, you have to add existence to the species dog.
And God is existence itself. Only of God can we say that His essence is his existence; for everything else, existence is something orthogonal to genus, species, and essence. And so, being existence itself, God cannot be a genus predicated of a number of species, because predicating existence of a species is alien to the notion of what a species is.